This application claims the priority of German patent document 199 43 248, filed Sep. 10, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a fuel cell system and a process for operating a fuel cell system.
Fuel cell systems can be used, for example, in fuel cell operated motor vehicles. A suitable liquid or gaseous fuel mixture, (e.g., a hydrogen rich gas mixture or a liquid mixture of water and methane) is fed into the anode region of the fuel cell where it is reacted, and the reacted mixture is then discharged via the anode output. Correspondingly, an oxygen-containing gas (air, for example) is fed into the cathode region, where the oxygen is reduced, thus forming water, and the water-containing cathode waste gas is exhausted via the cathode output.
In many applications it is desirable to separate the water contained in the cathode waste gas from the other components (e.g., in order to reutilize the water and/or the cathode waste gas, cleaned of the contained water). For this purpose there are water separators arranged in the cathode output. Usually they comprise a condenser, in which the water is removed by condensation at a suitably low temperature. This feature requires a substantial buildup of heat transfer for the condenser in order to adequately cool the cathode waste gas.
Often the cathode output also includes an expander in the form of a turbine that is coupled to a cathode input side compressor. In this case a condenser can be arranged upstream and/or downstream of the expander. Such a fuel cell system is described, e.g. in German patent document DE 197 01 560 A1.
The earlier German patent document DE 199 11 016 describes how to recover the water from cathode waste gas by means of a diaphragm. In this case, the temperature for separating the water does not have to be lowered substantially, but an adequately high pressure gradient from the mixture chamber, defining the one diaphragm side, to the water collecting chamber, defining the other diaphragm side, is required to achieve a satisfactory selective water separation from the residual cathode waste gas.
In a fuel cell system with low system pressure the problem of water separation becomes even more critical, since at low system pressure there is, on the one hand, no adequately high pressure differential for the diaphragm process, and, on the other hand, the requirements imposed on the temperature drop for water separation using a condenser, as compared to a high pressure system, increase. Thus, a suitable heat transfer buildup must be correspondingly even higher.
One object of the invention is to provide an improved process and apparatus for recovery in a fuel cell system at low pressure.
This and other objects and advantages are achieved by the fuel cell system and method according to the invention, in which the fuel cell is operated with a water balance, using a hydrogen-containing medium as the operating medium. The water requirement of a reforming reaction and/or the water balance of the fuel cell system is coupled directly to the oxidation rate of the partial combustion of the hydrogen-containing medium (or the mixture of mediums) in an oxidizing unit upstream of the reforming reactor.
At the same time, when the reforming reaction requires additional water, it is advantageous to increase the rate of oxidation, and to decrease it for a reduced water requirement.
Preferably the reforming reaction is supported directly with thermal energy generated during combustion. In particular it is preferred that the reforming reaction be supported thermally using only the thermal energy generated during combustion.
In a preferred embodiment of the invention, an evaporating reaction in a first evaporator is supported with the thermal energy generated during combustion.
In another preferred embodiment thermal energy from the waste gases of the fuel cell is provided for evaporating the hydrogen-containing medium or mix of mediums in a second evaporator so that the hydrogen-containing medium or the mixture of mediums is fed at least in part in a gaseous state to the first evaporator. If the reforming reaction requires additional water, it is advantageous to bypass the second evaporator.
In another preferred embodiment of the invention, carbon monoxide is removed from the reformate in an exothermic reaction, which is cooled independently of the thermal requirement of the reforming reaction.
In another preferred embodiment, thermal energy is also removed from a thermal system, comprised of a first evaporator, an oxidizing unit and a reforming unit or comprised of an oxidizing unit and a reforming unit, in order to increase the rate of oxidation.
It is advantageous to feed oxygen into the oxidizing unit as a function of the temperature in an area of combustion and/or in the area of the reforming reaction.
Another advantageous measure is to feed oxygen into the oxidizing unit as a function of the outlet temperature of the medium from the oxidizing unit and/or the outlet temperature of the reformate from the reforming reaction.
The apparatus for carrying out the process according to the invention comprises a fuel cell system, wherein an anode input includes an oxidizing unit for oxidizing a hydrogen-containing medium or a mixture of mediums and a reforming unit for generating a hydrogen rich reformate from the hydrogen-containing medium or mixture of mediums. The oxidizing unit is thermally coupled directly to the reforming unit.
It is also an advantageous embodiment to provide in the component a first evaporator for evaporating the hydrogen-containing medium (or mixture), with the evaporator being thermally coupled directly to the oxidizing unit.
In a preferred embodiment a second evaporator, which can be heated with the waste gases of the fuel cell, is arranged with respect to the hydrogen-containing medium or a mixture of mediums on the inflow side in front of the component.
It is advantageous to arrange a carbon monoxide removing unit between the component and the anode region, to remove carbon monoxide from the reformate. It is especially preferred that the carbon monoxide removing unit be coolable independently of the reforming unit.
The invention is especially appropriate for fuel cell systems with low system pressure, in particular for fuel cell systems in which at least the cathode output exhibits a medium pressure of a maximum of 2 bar. Such a low pressure level can be achieved with simple blowers, so that the cost of expensive compressors and other high pressure components can be saved. The system according to the invention is simple, and is especially advantageous for applications where the consumers are to be supplied with low to average electrical power using fuel cells.
It is particularly useful for auxiliary units in motor vehicles or stationary systems.